There are several applications for laser radiation projected to form a line of light on a target. Early versions of such line-projection apparatus for diode-lasers were found in bar-code readers and the like. Here, however, uniformity of illumination along the line was at most a secondary consideration.
Later versions of diode-laser line projection apparatus were developed for spatial light modulators (SLMs) in laser printers, image projectors, and the like. Here, power in the line of light was not required to be very high, and could be delivered by a simple one-dimensional laser array, commonly referred to as a diode-laser bar. Uniformity of illumination along the line of light in these applications is important. An “ideal” line of radiation requires a Gaussian intensity distribution of intensity in the width of the line and a uniform or “flat-top” distribution along the length of the line. This uniformity requirement encouraged the invention of projection optics that would provide adequate uniformity. Examples of such projection optics are described in U.S. Pat. No. 6,773,142, U.S. Pat. No. 7,016,393, U.S. Pat. No. 7,265,908, and U.S. Pat. No. 7,355,800, all of which are assigned to the assignee of the present invention.
A recent application for a projected line of diode-laser light is in patterning operations for organic light emitter displays. Here the line of light illuminates a mask having apertures defining emitter features for one primary color of the display. The illuminated features are projected by a projection lens onto a donor foil coated with a layer of organic emitting material for that color. This process sometimes referred to as laser induced thermal imaging (LITI). Exposed regions of the foil are transferred to an active matrix back-plate of the display. The process is repeated for building different primary colors of the display. The process is commercially more effective the longer the line of light that can be projected, and the higher the intensity of the light in the line. The intensity is dependent, for any given line-length, on the power of the illumination and the narrowness of the line
More laser power provides that a longer line can be projected at a given illumination intensity. Preferred line lengths for OLED manufacture greatly exceed the length of lines of light used for illuminating SLMs which are typically only about 25 millimeters (mm) long. A desirable line-of-light for OLED manufacturers would have a length of about 200 mm and a width of about 0.2 mm or less, with about 5 kilowatts (kW) total power delivered into the line. The line would preferably have Gaussian distribution of intensity in the line width, and a uniform (“flat top”) distribution in the line length. Depending on the optical efficiency illuminating apparatus, this would require a diode-laser source with a total power in excess of about 7 kW. As the power available from a typical multimode diode-laser bar is only on the order of 70 Watts (W), such a 7 kW-source would require about 100 diode-laser bars.
Each emitter has a height (in what is usually referred to as the fast-axis) of about 2 micrometers (μm) and a width (in what is usually referred to as the slow-axis) of about 100 μm. The ratio of the total width of emitters to the length of the diode-laser bar is usually referred to as the fill-factor. Each emitter delivers an astigmatic beam having a divergence in the fast-axis of about 35°, but a substantially Gaussian intensity cross-section, and a divergence in the slow-axis of about 10° but a more complex intensity cross-section. Two-dimensional arrays of diode-laser emitters have been made by stacking diode-laser bars in the fast-axis direction in diode-laser bar modules. However, each diode-laser bar must be individually cooled, and there is a practical limit to how many can be stacked in a module, dependent, inter alia, on selecting sufficient bars from a batch with adequate specifications. Certainly modules with 100 fast-axis stacked bars are not commercially available at present and it is believed that 100-bar stacks may not be available at economical cost, if at all for many years into the future.
It is to be expected that a multi-kilowatt diode-laser source will comprise some arbitrary array of such diode-laser-bar stacks, providing a two dimensional array of individual emitters. The emitters will be in a non-uniform, however symmetrical, distribution, with each emitter delivering a non-uniform beam with not necessarily the same non-uniformity. There is a need for an optical arrangement capable of delivering from such a source a line of radiation having a length of 200 mm or greater and a width of 0.2 mm or less with a Gaussian or near-Gaussian distribution of intensity in the line-width, and a uniform distribution of intensity in the line-length.